This invention relates to a radial flow turbine rotor for use in a supercharger or the like which uses a high temperature exhaust gas from an internal combustion engine as drive medium.
An exhaust gas supercharger is known which is used in an internal combustion engine to increase the density of air supplied for combustion and to raise the effective pressure of combustion gas. Most superchargers have a radial flow turbine rotor in a combustion exhaust gas passage. An ordinary radial flow turbine rotor comprises a shaft and precision-cast, heat-resistant steel blades welded to the periphery of the shaft. The maximum temperature that the radial flow turbine rotor withstands is about 650.degree. to 750.degree. C. The rotor is rotated at about 100,000 rpm, at most.
The lower portions of the blades which are welded to the shaft are likely to break when a high vibratory stress is applied on them as the rotor spins at a high speed. With the supercharger it is taken in a high temperature, high pressure exhaust gas, to rotate the radial flow turbine rotor at a higher speed and to reduce the stress acting on the blades as much as possible. To this end, the radial flow turbine rotor must be made of material which is light, mechanically strong and resistant to heat. The conventional heat-resistant steel is not satisfactory from this standpoint.
Recently ceramic turbine rotors have been developed. For example, a curved blade rotor made of ceramic material is shown at pages 888-891 of "CERAMICS FOR HIGH PERFORMANCE APPLICATIONS-II" published in 1978 by Brook Hill Publishing Company. The above-mentioned curved blade rotor was made by AME Ltd. in reaction bonded silicon nitride. The main object of making ceramic curved blade rotor is to replace expensive nickel alloys by cheaper, non-strategic materials and to operate the turbine at high temperatures. However, it has been found to be necessary to improve the design of the rotor in making a curved blade rotor of ceramic material.